So I am just getting around to the late game league stuff, because life, and I am sure glad they added another Sirus fight to the game. Double my least favorite content!!!
The leagues were the same way PoE was developed. A lot of things you see now in the core game date back to leagues from the betas. Stuff like strongboxes, exiles, a lot of the mods on mobs, half the bosses you fight in maps, shrines, onslaught, etc.